One reason for the increased need for storage is that the technology for generating electricity has changed Standard combined cycle power plants(CCPP) used more commonly can be described by FIG. 1a. These plants provide a large fraction of the electric energy in the world and their use is growing fast (reaching over 20% of installed capacity in the U.S.). During normal operation, compressed air is introduced to a combustion chamber with natural gas or other fuel introduced in a controlled way. The high temperature (1100-1200° C.) gases which are produced as a result of this combustion are introduced to gas turbine 110. The exhaust gases after the expansion in the gas turbine 110 have much lower pressure (barely sufficient above atmospheric pressure in order to flow). The temperature of these exhaust gases is 550-600° C. resulting in the exhaust gases carrying high grade energy, which is recovered in a heat recovery steam generator (HRSG) 120 by generated steam in a separate cycle. The design of the HRSG 120 is such that the resultant back pressure to the gas turbine is very low (40-50 mBars). Therefore, providing the HRSG does not affect the efficiency of gas turbine and introduces capability of operating a steam Rankine cycle from the recovered heat. The efficiency of modern combined cycle power plants is almost 55-60%. The control of the power output from the plant is governed by the demand in the grid and is, in turn, effected via supply and consumption of natural gas/fuel in the combustion chamber. This results in changed output from the gas turbine 110 and the steam turbine 130.
In conventional combined cycle power plants (CCPP), gas turbines operate continuously and the main control of electricity output is obtained by adjusting the main feed to the gas turbines. Since gas turbines have a very low turndown ratio, they can lose efficiency very rapidly when power is below maximum. The only control is basically on-off, as they can be shutdown in an hour and started up in one or two hours. The conventional CCPPs are not suitable for rapid load following for grid regulation and there is not enough overcapacity to enable such operation as a practical matter.
Therefore, it will be appreciated that there remains a need for new energy storage systems that can help address the above supply and control issues. There is also a need for storage systems that can operate while providing a resultant low back pressure.